This invention is related to a method for preparing nitrones, including diarylnitrones, dialkylnitrones and alkylarylnitrones. More particularly, this invention is directed to a method for preparing nitrones from a reaction mixture of arylhydroxylamines and/or alkylhydroxylamines obtained by the reduction of nitro compounds with zinc powder.
Arylhydroxylamines and alkylhydroxylamines are prepared by the reduction of the corresponding nitro compounds, i.e., nitroaromatic compounds and nitroalkanes. The reduction of such nitro compounds can be accomplished by a number of methods including electrochemical reduction, catalytic hydrogenation and metal reduction. Although catalytic hydrogenation often provides a high degree of selectivity and rate of reaction, as disclosed in copending application Ser. No. 777,390, reduction with metals may be desirable due to the simplicity of the process and the simple equipment which can be utilized to perform the reduction. Metal reductions are simple in that they need not take place under pressure or require a hydrogen source. Reduction of nitro compounds with metal powder, such as zinc powder, does pose problems in that the metal oxide coproduct often precipitates within the reaction mixture, necessitating a lengthy filtration step to remove the metal oxide.
It is desirable to avoid this lengthy filtration step since arylhydroxylamines are known to undergo side reactions over time due to their high reactivity. For example, in producing diarylnitrones from the reaction of arylhydroxylamine and arylaldehyde, it is advantageous to commence the reaction immediately upon formation of the arylhydroxylamine. However, this cannot be accomplished where the residual metal oxide remains as a voluminous precipitate in the reaction mixture.